Fabrication of semiconductor devices often involves formation of a dielectric layer. For example, in order to make a metal-oxide-semiconductor (MOS) transistor, a gate oxide is generated on a substrate. Conventionally, gate oxides are sometimes formed using in situ steam generation, where pre-mixed H2 and O2, for example, are introduced to a process chamber to flow across a rotating substrate heated by tungsten-halogen lamps to generate oxides on the substrate.
An integrated circuit (IC) chip often includes core devices for performing certain functions and input/output (I/O) devices for communicating with external circuits. Transistors used for core devices and I/O devices are often different. For example, an I/O device usually has a relatively thick gate oxide in order to sustain a high voltage applied to the peripheral circuit. On the other hand, a core device often requires a very thin gate oxide to achieve a strong capacitive effect for lower threshold voltage and good current control. Using in-situ steam generation to fabricate gate oxides of core devices and I/O devices on a same IC chip often involves forming gate oxide layers with different thicknesses and multiple cleaning stages, which often results in high fabrication costs.